Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y47: Spin Transfer Torque and Magnetic Tunnel JunctionsFocus
|
Hide Abstracts |
Sponsoring Units: GMAG DMP FIAP Chair: Chunhui Du, Harvard University Room: 394 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y47.00001: Current dependence of spin-wave excitation by spin transfer in magnetic point contacts Tucker Hartland, Andrei Zholud, Ryan Freeman, Sergei Urazhdin Spin transfer torque (STT) effect enables generation of spin waves in nanomagnetic systems, due to the transfer of angular momentum from spin-polarized electrical current to magnetization. The existing models are generally based on the assumption that all the spin wave modes can be excited by STT, irrespective of the energies of the scattered electrons [1]. We present low-temperature electronic spectroscopy measurements in various geometries of point contacts based on the GMR magnetic multilayers. Our measurements show anomalous dependence of resistance on the electrical current and on the magnetic field, indicative of the dependence of magnon generation rates on the energies of electrons involved in STT. This conclusion is further supported by the temperature dependence: the observed features disappear with increasing temperature, which is consistent with the thermal broadening of the electron energy distribution. We discuss the dependence of the observed effects on the geometry and structure of the magnetic systems, and the implications of these observations for our understanding of the current-induced magnetization dynamics in nanomagnetic systems. [1] J.C. Slonczewski “Current-driven excitation of magnetic multilayers”, J. Magn. Magn. Mat. 159, L1-L7 (1996) [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y47.00002: Voltage control of ferromagnetic resonance in magnetic tunnel junctions: perpendicular vs in-plane magnetization M. Williamson, H. Seinige, H. Almasi, X. Chao, W. Wang, J.-P. Wang, M. Tsoi Voltage controlled magnetic anisotropy (VCMA) attracts considerable attention as a novel method to control and manipulate magnetizations in fast-switching and low-power spintronic devices based on magnetic tunnel junctions (MTJs). In our experiments, we probe VCMA by ferromagnetic resonance (FMR) driven by microwave currents applied to CoFeB/MgO/CoFeB MTJs subject to high dc biases. We compare the effect of voltage bias on FMR in MTJs with perpendicular and in-plane magnetizations. As expected, we observe a linear shift of the resonance field with the applied bias in perpendicularly magnetized samples. In contrast, the in-plane samples exhibit a quadratic shift of the resonance field as a function of the dc bias. Both shifts can be explained by changes in the effective field due to the onset of out-of-plane VCMA. This work was supported in part by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y47.00003: Ab initio calculations of bias voltage dependence of magneto crystalline anisotropy in magnetic tunnel junctions Christian Heiliger, Carsten Mahr, Michael Czerner Spin-orbit effects play an important role in current spintronics research. One effect due to spin-orbit coupling is the magneto crystalline anisotropy (MCA) and the control of this effect by a bias voltage. Using density functional theory in combination with non-equilibrium Green’s function method we calculate the bias voltage dependence of MCA for the case of a V/Fe/MgO/V. We discuss the dependence of MCA and of the tunneling anisotropic magneto resistance (TAMR) on the Fe and MgO slab thicknesses. Further, we show the voltage dependence of spin-torque originated in these tunnel junctions and clarify the connection to the MCA. All our results are compared to recent experimental results in the same junctions. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y47.00004: Low switching current in tungsten-based magnetic tunnel junctions with an ultrathin Hf insertion layer Shengjie Shi, Yongxi Ou, Daniel Ralph, Robert Buhrman The discovery of a strong spin Hall effect (SHE) in certain heavy metals (HM) provides an efficient way to manipulate magnetization at the nanoscale with an in-plane electric current. Beta phase tungsten is reported to have a very large spin Hall angle (\textasciitilde 0.3) compared to the HMs (Pt or Ta) that have been more widely used in three terminal magnetic tunnel junctions (MTJ). However, due to difficulty in material stack development and device fabrication procedure, limited work has been done with tungsten-based MTJ systems. Here we report on our development of a reliable procedure for making tungsten-based MTJs and on the notable critical current reduction achieved by the insertion of an ultrathin Hf dusting layer between the FeCoB free layer and MgO tunnel barrier that can effectively reduce the demagnetization field by enhancing interfacial perpendicular anisotropy at the FeCoB/MgO interface. With material stack optimization we have to date obtained a critical switching current density of 6.5x10\textasciicircum 10A/m\textasciicircum 2. We also report fast, ns timescale, and reliable pulse switching results of tungsten-based MTJs, which confirms low switching current and demonstrates the potential for application in energy efficient memory cells. [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y47.00005: Spin transfer due to zero-point magnetization fluctuations Andrei Zholud, Ryan Freeman, Rongxing Cao, Ajit Srivastava, Sergei Urazhdin Spin transfer torque (STT) enables efficient control of the magnetization state and generation of spin waves in nanomagnetic systems by spin-polarized electrical current, which is valuable for applications in spintronic devices. So far, STT has been interpreted exclusively in terms of the classical states of the magnetization interacting with spin current. We present theoretical analysis and experimental results demonstrating the significance of quantum magnetization fluctuations in STT. We will discuss a simple model describing the scattering between an electron spin and a quantum macrospin representing the magnetization. Our quantum scattering model predicts that the quantum fluctuations result in a non-analytical piecewise-linear dependence of spin wave intensity on the electrical current, with a singularity at zero current, and spin-wave generation by quantum STT for both directions of the spin-polarized current. Experimental measurements of the standard giant magnetoresistive nanopillars, based on Permalloy/Cu/Permalloy spin valves, confirm that the predicted quantum effects are dominant at modest cryogenic temperatures. Our results demonstrate that the entire spin-wave spectrum is involved in STT, with an average frequency of the excited spin waves in the THz range. [Preview Abstract] |
Friday, March 17, 2017 12:15PM - 12:27PM |
Y47.00006: Enhancing spin transfer torques in magnetic tunnel junctions by ac modulation Xiaobin Chen, Chenyi Zhou, Zhaohui Zhang, Jingzhe Chen, Xiaohong Zheng, Lei Zhang, Can-Ming Hu, Hong Guo Phenomena of spin transfer torques (STTs) have been attracting persistent interests due to promising prospects of STTs in designing nano devices. By using nonequilibrium Green's function method, we derive time-averaged formulism for spin transfer torques in a noncollinear magnetic tunneling system under ac modulation. Using these formulas, we further investigate ac spin transfer torques in a carbon-nanotube-based magnetic tunneling system. It shows that under ac modulation, the low-bias linear (quadratic) dependence of the in-plane (out-of-plane) torque on bias still holds, and the $\sin\theta$ dependence on noncollinear angle is maintained. By introducing photon-assisted tunneling, the bias-induced components of the in-plane and out-of-plane torques can be enhanced by about 12 and 75 times, respectively. Further analysis shows that optimized enhancement can be achieved by using ac driving frequency $\omega=\epsilon_0/k, k=1,2,...$, where $\epsilon_0$ marks a remote dc transmission peak, and ac amplitude $\Delta$ such that $\Delta/\omega$ maximizes a $k^{th}$-order Bessel function. Our findings suggest that ac modulation is an effective way for electrical manipulation of STTs, paving the road towards emerging STTs-based nanoelectronics and spintronics. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y47.00007: Spin torque and shot noise in a ferromagnet-antiferromagnet tunnel junction Georg Schwiete, Kei Yamamoto, Helen Gomonay, Jairo Sinova We study the junction between a ferromagnetic and an antiferromagnetic metal connected by a weak tunneling barrier. When a current is driven through the junction from the ferromagnet, a spin torque acts on the antiferromagnet and shot noise arises in connection with the current. To describe these phenomena theoretically, we derive an effective stochastic Landau-Lifshitz-Gilbert equation for the antiferromagnet in the macrospin approximation. In this talk I will discuss the different terms appearing in the equation as well as their microscopic origin. [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y47.00008: Microwave oscillations in magnetic tunneling junctions with perpendicular anisotropy in the absence of bias magnetic field Rajapaksayalage Rajapakse, Zhongming Zeng, HongWen Jiang Microwave oscillators based on spin-transfer-torque are emerging as promising high-frequency spintronic devices.~ We report our investigation of microwave generation and spin dynamics of magnetic tunneling junctions with perpendicular anisotropy.~ For our experiments, a Fe-rich free layer is used in the magnetic film stack to enhance the perpendicular anisotropy. Circular shaped nano-pillar is patterned to induce a magnetic vortex state.~ We find that our devices produce stable microwave oscillations for frequencies up to 5 GHz in zero bias magnetic field. The threshold DC current for microwave emission as well as the spectra linewidth are considerably smaller than that of the in-plane MTJ structures. The current dependence of the frequency is almost symmetric about both current directions. We will also report the effects of parametric excitation in our devices using microwave current injection. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y47.00009: Introducing 2D Materials for Magnetic Tunnel Junctions Maelis Piquemal-Banci, Regina Galceran, Florian Godel, Marie-Blandine Martin, Sabina Caneva, Robert Weatherup, Stephan Hofmann, Stephane Xavier, Richard Mattana, Abdelmadjid Anane, Frederic Petroff, Albert Fert, Bruno Dlubak, Pierre Seneor We will present here experimental results on 2D materials integration in Magnetic Tunnel Junctions. A thin graphene passivation layer, directly integrated by low temperature catalyzed chemical vapor deposition (CVD), can prevent the oxidation of a ferromagnet [1]. This in turn enables the use of novel humide/ambient low-cost processes for spintronics devices. We will illustrate this property by demonstrating the use of ozone based ALD processes to fabricate efficient spin valves protected with graphene [2]. Importantly, the use of graphene on ferromagnets allows to preserve a highly surface sensitive spin current polarizer/analyzer behavior and adds new enhanced spin filtering property [3]. Finally, we will present results concerning the atomically thin insulator hexagonal boron nitride (h-BN) [4]. These different experiments unveil promising uses of 2D materials for spintronics. [1] Dlubak ACS Nano 6, 10930 (2012), Weatherup ACS Nano 6, 9996 (2012) ; [2] Martin ACS Nano 8, 7890 (2014) ; [3] Martin APL 107, 012408 (2015) ; [4] Piquemal-Banci APL 108, 102404 (2016) [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y47.00010: Multi-parameter geometrical scaledown study for energy optimization of MTJ and related spintronics nanodevices I. A. H. Farhat, C. Alpha, E. Gale, D. Y. Atia, A. Stein, A. F. Isakovic The scaledown of magnetic tunnel junctions (MTJ) and related nanoscale spintronics devices poses unique challenges for energy optimization of their performance. We demonstrate the dependence of the switching current on the scaledown variable, while considering the influence of geometric parameters of MTJ, such as the free layer thickness, $t_{free}$, lateral size of the MTJ, $w$, and the anisotropy parameter of the MTJ. At the same time, we point out which values of the saturation magnetization, $M_{s}$, and anisotropy field, $H_{k}$, can lead to lowering the switching current and overall decrease of the energy needed to operate an MTJ. It is demonstrated that scaledown via decreasing the lateral size of the MTJ, while allowing some other parameters to be unconstrained, can improve energy performance by a measurable factor, shown to be the function of both geometric and physical parameters above. Given the complex interdependencies among both families of parameters, we developed a particle swarm optimization (PSO) algorithm that can simultaneously lower energy of operation and the switching current density. Results we obtained in scaledown study and via PSO optimization are compared to experimental results. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y47.00011: Enhanced nanomagnet spin-transfer-toque reversal through strain-induced meta-stable magnetization initialization Nickvash Kani, Azad Naeemi Nanomagnet reversal through constant longitudinal spin currents is typically a slow and energy intensive process due to the fact that only the transverse component of the spin current applies a toque on the nanomagnet and hence, this torque is very small if the magnetization is initially along the free-axis. Recent research has shown that a piezoelectric material can be placed under a nanomagnet and introduce a strain-induced anisotropy initializing the magnetization to any orientation. It has been previously shown that the probability that a nanomagnet will reverse to the parallel energy basin under a constant spin current is a single exponential function and that regardless of initial magnetization, the spin current magnitude must be greater than the critical current value to ensure reliable coupling. However, strain-induced magnetization initialization can provide some energy savings when reversing the nanomagnet. It is shown that in the meta-stable (y) initialization case, reliable reversal is achieved for very small current pulses. Hence, initializing nanomagnets to a meta-stable state through piezo-electric materials can be a useful method for developing fast and energy efficient spin-based devices. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y47.00012: Correlation between the direction of exchange bias and spin transfer torque in metallic antiferromagnets/Permalloy bilayers Hilal Saglam, Wei Zhang, M. Benjamin Jungfleisch, John E. Pearson, Axel Hoffmann Recent work shows that magnetic precession in a ferromagnet (FM) can be excited by spin transfer torque arising from spin Hall effect in metallic antiferromagnets (AF) [1]. However, in these measurements the AF was separated from the FM by a thin copper layer to avoid direct exchange coupling. Here we investigate spin transfer torques in exchange biased systems where a hysteresis loop is shifted relative to the zero-field position due to the exchange interaction between AF and FM [2]. We use spin torque ferromagnetic resonance measurement on AFs/Py (Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}})$ bilayers in order to investigate the effect of exchange bias (EB) on spin transfer torques. For that purpose, we perform field-cooling experiments with an applied field parallel and perpendicular to the sample plane and compare the resultant spin Hall conductivities. Interestingly, we observed a unidirectional behavior, where the current induced torques are inequivalent for opposite field directions. [1] W.~Zhang \textit{et al}., Phys. Rev. B \textbf{92}, 144405 (2015). [2] J.~Sklenar \textit{et al}., AIP Adv. \textbf{6}, 055603 (2016). [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y47.00013: Abstract Withdrawn |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700